Roundup and Cancer

International regulators and researchers disagree on whether Roundup and other glyphosate-based herbicides increase the risk of cancer.

Studies of cancer rates in people exposed to glyphosate have returned conflicting results, according to the National Pesticide Information Center. But the center says some studies have associated glyphosate-based herbicides with non-Hodgkin lymphoma.

Research into an association between glyphosate and cancer in humans is very limited. Studies have shown large doses of the chemical may cause cancer in animals. But results from animal tests don’t always translate to the same results in humans.

Drug and chemical giant Bayer AG acquired Roundup’s original manufacturer, Monsanto, in 2018. Bayer’s glyphosate website touts “more than 800 rigorous studies” on glyphosate conducted for United States, European and other regulators. It says these studies confirm glyphosate is not carcinogenic. But company emails from 2002 suggest no studies at that time had determined whether Roundup posed a cancer risk.

A 2019 analysis published in the journal Mutation Research/Reviews in Mutation Research said evidence supports a link between exposure to glyphosate herbicides and a higher risk of non-Hodgkin lymphoma, a specific type of cancer.

The researchers conducted a meta-analysis of all published studies of glyphosate exposure in humans. They determined people who had been exposed to the largest amounts of the herbicide showed a 41 percent higher relative risk for the disease.

“Our current meta-analysis of human epidemiological studies suggests a compelling link between exposures to [glyphosate-based herbicides] and increased risk for [non-Hodgkin lymphoma],” the authors wrote.

The research included a fresh look at 2018 data from the government-funded Agricultural Health Study. Monsanto had used that study to argue there was no association between glyphosate and the disease.

The five United States researchers who wrote the 2019 analysis included three scientists who served on a 2016 Environmental Protection Agency advisory committee on glyphosate. The chief editor of the journal that published their work is also an EPA scientist.

The study’s findings differed sharply to those of other researchers who had examined Agricultural Health Study data just two years earlier.

A 2016 study in the British Medical Journal found only one cohort study had looked into a cancer association. Its results showed no apparent association between exposure and cancer risk. But the average follow-up time with participants in the study was 6.7 years, which is too short of a period to account for the time between initial exposure and when a doctor diagnoses cancer.

Following an extensive review of published research on glyphosate, the International Agency for Research on Cancer concluded in March 2015 that the herbicide is “probably carcinogenic.” The agency is part of the United Nations’ World Health Organization.

Its statement noted only “limited evidence” that glyphosate may cause cancer in humans. The agency said it based its decision on “sufficient evidence” that the chemical had caused cancer in tests on animals.

The agency had also looked at other research that had found strong evidence that glyphosate-based pesticides can damage genes. This is called genotoxicity and can lead to cancer.

The 2015 report stressed the amount of exposure and the way the chemical gets into the body play major roles in whether a person develops cancer.

The Environmental Protection Agency says Roundup and other glyphosate-based herbicides present “low toxicity” to humans, but the agency recommends people wait 12 hours before entering fields that have been treated with the chemical.

The EPA in 2021 said it found that glyphosate is “unlikely to be a human carcinogen.” The regulator recommends management measures such as targeting pesticides for specific pests, make weeds less resistant to glyphosate and protect plant pollinators.

“The agency’s assessment found no other meaningful risks to human health when the product is used according to the pesticide label,” the EPA said in a statement.

But departments within the agency have disagreed over Roundup’s potential to cause cancer.

In 2015, the EPA’s Office of Pesticide Programs re-classified the weed killer as “not likely” to cause cancer. The agency’s Office of Research and Development questioned the decision. It recommended the Office of Pesticide Programs strengthen its human health assessment and explain why it came to a different conclusion than the International Agency for Research on Cancer.

The European Food Safety Authority determined in November 2015 that glyphosate is “unlikely to pose a carcinogenic hazard to humans.” The regulatory body for the European Union examines and regulates pesticide use as part of its mission.

The agency’s conclusion came just eight months after the International Agency for Research on Cancer branded glyphosate as “probably carcinogenic.” The chief difference between the two reports is the UN agency considered the glyphosate association with non-Hodgkin lymphoma to have “limited evidence in humans” while in the EU assessment, most experts considered it “very limited.” The one word difference in descriptions was enough to prevent the European Food Safety Authority from declaring a cancer risk classification.

The European Food Safety Authority included the International Agency for Research on Cancer report in its research after the European Commission requested it look at the UN data.

“The evaluation considered a large body of evidence, including a number of studies not assessed by the [International Agency for Research on Cancer] which is one of the reasons for reaching different conclusions,” the European Food Safety Authority said in a news release.

All but one member of the panel that had examined glyphosate agreed that neither human nor animal studies showed a link between glyphosate exposure and cancer in humans. But the European agency did set new safe exposure levels for people who work around glyphosate or eat food that contains herbicide residue.

In 2017, California environmental regulators listed glyphosate as “known to the state to cause cancer.” The state’s Office of Environmental Health Hazard Assessment made the decision based in part on the report from the International Agency for Research on Cancer. State Proposition 65 requires the state office to add all substances the international agency deems carcinogenic to a state list of cancer-causing items.

The state office relies on several agencies to determine whether to add a substance to its carcinogenic warning list. The agency’s website says two other sources it relies on are in the process of reviewing glyphosate’s cancer risk. These are the Environmental Protection Agency and the National Toxicology Program, which is part of the United States Department of Health and Human Services.

The agency says several organizations have determined glyphosate is not likely to cause cancer. But it also says “scientific disagreement on the interpretation of data is not unusual and does not affect the listing.”

California regulators went a step further, however. They also required all food products that contain traces of glyphosate to carry a warning label.

Large agriculture industry groups challenged the legality of the food warning labels. In February 2018, a federal judge temporarily blocked the labels.

Monsanto had also sued the state to remove glyphosate from the state’s warning list. But in April 2018, a federal appeals court said it could remain on the list.